1. Field of the Invention
The present invention relates to a zoom lens system, for a single-lens reflex (SLR) camera, having a suitable zoom ratio of approximately 4, and having an angle-of-view of over 80° at the short focal length extremity.
2. Description of the Prior Art
As a zoom lens system, a negative-lead type zoom-lens system has generally been employed in the case where a zoom ratio of up to approximately 3 is required. If the zoom ratio exceeds approximately 4, a positive-lead type zoom-lens system including four or more lens groups has been employed.
However, a zoom lens system in which a zoom ratio, an angle-of-view, a smaller open F-number which does not vary upon zooming, and an optical quality thereof are well balanced has not been known.
For example, in Japanese Unexamined Patent Publication (JUPP) No. 2004-354980,a zoom lens system having four lens groups, i.e., a lens group having a negative refractive power (hereinafter, a negative lens group), a lens group having a positive refractive power (hereinafter, a positive lens group), a negative lens group, and a positive lens group, is proposed. The zoom lens system has an angle-of-view exceeding 80° at the short focal length extremity, and achieves an open F-number 4 (constant) over an entire range from the short focal length extremity to the long focal length extremity; however, the zoom ratio thereof is only a little under 3.
In JUPP No. 2005-107262,a zoom lens system having five lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a negative lens group, and a positive lens group, is proposed. The zoom lens system has the open F-number which varies in accordance with the focal length is large (i.e., 4.6) at the long focal length extremity. If an attempt is made to achieve an open F-number of 4,spherical aberration and coma are undercorrected.
In JUPP Nos. 2006-227526 and 2004-233750, zoom lens systems include five lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a negative lens group, and a positive lens group. The zoom lens systems have a zoom ratio exceeding 4,and have an angle-of-view exceeding 80° at the long focal length extremity; however, this angle-of-view is equivalent to the angle-of-view of an imaging device with the 35 mm-film-format size, and is not sufficient for a smaller imaging device. If an attempt is made to simply make the focal length of the zoom lens system shorter (scaling) in order to widen the angle-of-view, the back focal distance cannot be secured. Furthermore, in these publications, the open F-number which varies in accordance with the focal length is 4.8 and 5.9 at the telephoto extremity; however, these open F-numbers are too large from the viewpoint of the balance among the angle-of-view, the open F-number and an optical quality.
In JUPP No. 2006-234892, a zoom lens system includes five lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a negative lens group, and a positive lens group. An open F-number of 2.8 which does not vary in accordance with the focal length is sufficiently bright (fast); however, the zoom ratio is smaller, i.e., approximately 3. If the zoom ratio is increased, the correcting of spherical aberration and coma becomes difficult.
In JUPP No. 2005-352057, zoom lens systems include four to six lens groups with various refractive power distribution over the lens groups; and, an open F-number which varies in accordance with the focal length is approximately 5.8 at the long focal length extremity, which is large from the viewpoint of the balance among the angle-of-view, the open F-number and an optical quality. On the other hand, in the case where an open F-number which does not vary in accordance with the focal length is 4.1, a sufficient zoom ratio cannot be achieved.